feat(llm): add BYOK tier + 4 provider adapters (OpenAI, Anthropic, Gemini, Mistral)

Phase 1-3 of BYOK support. Introduces a 5th LLM tier 'byok' that
routes to user-provided API keys via direct browser fetches.

shared-llm additions:
- LlmTier extended with 'byok' (rank 3, between mana-server and cloud)
- ByokBackend: LlmBackend implementation that delegates key lookup
  to an app-provided resolver callback, then dispatches to the right
  provider adapter
- 4 provider adapters:
  - OpenAI (gpt-5, gpt-4o, o1 family)
  - Anthropic (Claude Opus/Sonnet/Haiku 4.6) with CORS header
  - Gemini (2.5 Pro/Flash) — REST API with different message format
  - Mistral — OpenAI-compatible, reuses shared openai-compat adapter
- Pricing table for 20+ models with USD per 1M tokens
- estimateCost() + formatCost() helpers

Keys stay device-local (IndexedDB in next phase). Browser-direct
fetches mean keys never touch Mana's server.

Updates two existing tier maps (memoro DetailView, SourceBadge) to
include the new tier.

Planning doc at docs/architecture/BYOK_PLAN.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-llm/src/index.ts

@@ -44,6 +44,26 @@ export { LlmOrchestrator, type LlmOrchestratorOptions } from './orchestrator';
 export { BrowserBackend } from './backends/browser';
 export { CloudBackend, type CloudBackendOptions } from './backends/cloud';
 export { ManaServerBackend, type ManaServerBackendOptions } from './backends/mana-server';
+export {
+	ByokBackend,
+	type ByokBackendOptions,
+	type ByokKeyResolver,
+	type ResolvedByokKey,
+	type ByokUsageCallback,
+} from './backends/byok';
+export {
+	BUILTIN_BYOK_PROVIDERS,
+	openaiProvider,
+	anthropicProvider,
+	geminiProvider,
+	mistralProvider,
+	type ByokProvider,
+	type ByokProviderId,
+	type ByokCallOptions,
+} from './backends/byok-providers';
+
+// Pricing
+export { MODEL_PRICING, estimateCost, formatCost, type ModelPricing } from './pricing';
 
 // Singleton store + Svelte 5 reactive hooks
 export {